This is a direct violation of the principle of Suggestion.

Such a command only tends to empty the pupil's mind of other objects of thought and interest, and so to keep his attention upon his own movements.

This, then, amounts to a continual suggestion to him to do just what you want to keep him from doing.

On the contrary, unless you give him suggestions and interests which lead his thought away from his acts, it is impossible not to aggravate his bad tendencies by your very efforts.

This is the way, as I intimated above, that many teachers create or confirm bad habits in their pupils, and so render any amount of well-intended positive instruction abortive.